growing concerns this morning over security as the american team now prepares to head to russia for the winter olympics just weeks away. We're getting a clearer picture today of the explosions. The two deadly bombings in two days. Brian ross, back from sochi, where he pressed authorities there. Reporter: U.S. Olympic officials are saying this morning they have full confidence that russian officials will make sure the olympic games are safe. But counterterrorism officials tell abc news, they worry that the two blasts this week, may be a wave of terror attacks that will only increase as the games get closer. Olympic security officials today will continue to pore over the chilling our surveillance tapes from volgograd, including this video from two news agencies, they say shows two suspects at the train station checkpoint before the suicide bomb was detonated. In six weeks, this will be a hub for athletes and spectators, heading to the olympics. This is one of the train stations they have to pass through on their way to moscow down to sochi. Reporter: Sochi, the site of the winter games is 400 miles away. President vladimir putin has promised air-tight security. And the mayor of sochi told me earlier this month, he had no worries about terrorism. No worries. But not all u.S. Officials share that confident view. Given this islamic terrorist leader and his group to disrupt the olympics. They're battle-trained. They're battle-ready. They have capability. Reporter: And some officials tell abc news, the russians have failed to fully cooperate with american security teams, assigned to protect the 235 u.S. Athletes who will come here to sochi. Officials told me when I was in sochi earlier this month, they plan to shut down all traffic but for official cars and ring the city with troops and security officials. In their pledge to make it air-tight from terrorists. A fortress. Right. Now, to the dramatic final

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Deadly Bombings in Russia Ahead of Winter Olympics

Dozens reportedly killed in twin explosions in Volgograd, Russia.

Terror Attacks in Russia: Significance of Volgograd

Major Russian city, formerly called Stalingrad, is close to the volatile North Caucasus region.